Jun-Hee Hahn

Identification of the Γ5 and Γ6 free excitons in GaN

The Γ5 and Γ6 free excitons have been identified in GaN from emission measurements. Another emission peak is also observed which we believe to be the longitudinal free exciton. These measurements along with electrical measurements, which show the sample to have very high peak mobility, attest to the high quality of the sample.

Effect of Interfacial Adhesion on the Mechanical Properties of Organic/Inorganic Hybrid Nanolaminates

Two different kinds of organic polyelectrolyte (PE)/inorganic silicate nanolaminates carrying dissimilar interfacial adhesion between the organic and the inorganic layers were prepared using the layer-by-layer self-assembly. To investigate the mechanical behavior of the prepared hybrid films, apparent modulus (E′), hardness (H), and crack length were measured by depth-sensing nanoindentation as well as a microVickers experiment. The fracture toughness of the hybrid films was then calculated based on the measured mechanical values. In the case of forming strong interfacial adhesion between the organic and the inorganic layers (A series), the fracture toughness and the crack resistance of hybrid multilayer films were significantly improved as a result of the redistribution of stress concentration and the dissipation of fracture energy by the plasticity of organic PE layers. On the other hand, samples with relatively low interfacial adhesion between the organic and the inorganic layers (T series) had little effect on the improvement of fracture toughness of the hybrid films.

Effect of UV Pretreatment on the Nanopore Formation within Organosilicate Thin Films

We have investigated the low-temperature cure process to realize nanoporous organosilicate thin films at temperature below 150°C by adding a small amount of photoacid generator (PAG) followed by UV irradiation. The Gemini surfactant, which decomposes in the temperature range from 170 to 420°C, was used as a pore-generating material (porogen) for organosilicate matrix. The UV pretreatment in the presence of PAG lowers the condensation temperature of poly(methyl silsesquioxane) matrix and leads to the fast matrix vitrification enabling the addition of increased amount of porogens. Because the full vitrification of the matrix (150°C) by UV pretreatment in the presence of PAG below the decomposition temperature of porogens (170°C) prevents the pore collapse, the porosity up to 35.5% was achieved with an average pore size of 3.4 nm, as measured from X-ray reflectivity as well as ellipsometric porosimetry. It is shown that both dielectric constant and refractive index continue to decrease to 2.0 and 1.26, respectively. The present experimental system demonstrates that porogens with low degradation temperature can be successfully incorporated to realize nanoporous films without pore collapse. Consequently, this process can widen the choice of porogens to prepare nanoporous films.

Effect of Pore Generating Materials on the Electrical and Mechanical Properties of Porous Low-k Films

Two types of porogens were tested to prepare porous low dielectric films. PCL porogens as well as Tetronic block copolymer porogens generate pores within the films thus lowering the dielectric constant. Tetronic porogens, however, yield porous films without significant matrix-porogen interactions and thus result in higher mechanical properties with lower dielectric constant, when compared with the PCL porogens.